The present invention relates to a tube support grid and spacers therefor, and has particular application to the design of tube support grids for heat exchangers, especially heat exchangers used in nuclear reactors.
While the present invention is of general applicability, it contemplates a gas/liquid heat exchanger with generally vertically extending heat exchange tubes. A heat exchanger of the type herein considered includes a heat exchange chamber through which superheated helium, or other coolant gas, flows adjacent the outer surfaces of heat exchange tubes. Water, or other liquid coolant, flows within the tubes so as to absorb heat from the helium. The water enters the tubes from a water inlet chamber through a tube sheet to which the tubes are welded.
A tube support grid is often used to support the upper ends of the tubes. Normally, a tube support grid is spaced some distance down from the tops of the tubes. Preferably, the grid provides support for the tubes, maintains appropriate spacing between respective tubes, and provides relief from vibrations which might be generated from seismic or other events.
The temperature differentials in such a heat exchanger can be substantial. These differentials are particularly large in heat exchangers used to dump heat from a reactor core during reactor malfunction. Gas inlet temperatures exceed 1700.degree. F., while water inlet temperatures are generally less than 200.degree. F. These temperature differentials can generate stresses due to differential thermal expansion. In the contemplated heat exchanger, the expansion of the tube sheet is governed primarily by the inlet temperature of the water, while the expansion of the tube support grid is governed primarily by the temperature of the gas. Consequently, the grid tends to expand more than the tube sheet.
Where the tubes are vertical under normal conditions (e.g. at room temperature, or at normal temperatures when the heat exchanger is not in use), differential expansion of the tube sheet and grid causes the tops of the tubes to be forced radially outward relative to their bases. This tilting stresses the base welds. Eventually the stresses will break the welds, perhaps allowing liquid to flow into the exchange chamber with dire effects. Safety and economic considerations require that the stresses on the welds be minimized.